1. Field of the Invention
The present invention relates to a method for incorporating into zeolite pores dipolar nonlinear optical (DNLO) molecules in a uniform orientation and a DNLO-zeolite composite in which DNLO molecules are included in a uniform orientation into zeolite pores.
2. Description of the Related Art
Incidence of laser beams with a frequency of ω into second-order nonlinear optical materials results in the occurrence of harmonic wave with a frequency of 2ω. This phenomenon has been applied to the development of optical switch, optical sensor and devices generating new laser beams with short wavelength.1-4 A wide variety of inorganic crystals are used as second-order nonlinear optical materials. However, the inorganic crystals have serious problems: less feasibility to prepare crystals of a suitable size; and slower response time due to high dielectric constant. Therefore, attempts have been extensively made to replace inorganic crystals with organic materials with second order nonlinear optical properties.1-4 
Various organic molecules with second order nonlinear optical properties have been suggested and their representative is a dipolar nonlinear optical (DNLO) molecule expressed by D-π-A in which an electron donor portion having relatively rich electrons is conjugated to an electron acceptor portion having relatively deficient electrons.1-4 Even though a single DNLO molecule exhibits significantly high value of second-order hyperpolarizability constant that becomes increased with help of molecular design, various optical devices such as optical switch cannot be manufactured using a single DNLO molecule.
Therefore, it could be appreciated that only crystals or aggregates comprises of DNLO molecules aligned in one direction (orientation) are useful in the development of optical devices. However, upon crystallizing, most of D-π-A molecules show the strong alignment tendency to counterbalance dipole moments each other. Hitherto, a lot of attempts have been made to prepare crystals of DNLO molecules aligned in one direction but most of them have been unsuccessful.1 
One of approaches having been suggested, which is generally called electric poling, is to align DNLO molecules in one direction by imposing strong electric field on DNLO molecules incorporated homogeneously in a polymer medium at glass transition temperature of the polymer.2 
The incorporation of DNLO molecules into a polymer medium is executed by a physical mixing process or a chemical process in which DNLO molecules are covalently linked to a polymer chain. However, poled polymers of nonlinear optical molecules show low second-order nonlinear optical sensitivity per unit volume, expressed by d33 (a tensor component of the quadratic nonlinear susceptibility), regardless of the type of incorporation methods. Furthermore, the alignment of DNLO molecules incorporated is very likely to be spontaneously disrupted with the lapse of time even at relatively low temperature, rendering applicability of the poled polymers to be sharply decreased.3 
Much effort has been directed at utilizing DNLO molecules as self-assembled mono- or multilayers4-8 and polymer-DNLO composite films supported on optically transparent substrates.1,2,9-13 However, although the organic thin films of DNLO molecules show significantly high second-order nonlinear optical sensitivity per unit volume, their thickness is very little and their thermal and mechanical strength are far poor, making their practicability negligible. The disadvantages associated with the organic thin films of DNLO molecules have triggered scientists for other novel conceptual approaches in the use of DNLO molecules.
Along this line, zeolites and the related nanoporous materials have been examined as hosts for aligned incorporation of DNLO molecules to explore novel organic-inorganic composites.14-22 For example, Stucky and the coworkers first reported that DNLO molecules such as para-nitroaniline (PNA), 2-methyl-4-nitroaniline (MNA), 2-amino-4-nitropyridine, and the analogous compounds readily enter the straight channels of AlPO4-5 [a noncentrosymmetric (P6cc) zeolite analog having one-dimensional channels with the diameter of 0.8 nm], and the dye-incorporating AlPO4-5 powders generate second harmonic (SH) with the intensity far exceeding that of quartz powders.14,15 
Subsequent studies by Marlow, Caro and their coworkers revealed that the SHG activity of the PNA-including AlPO4-5 crystals arose as a result of the spontaneous incorporation of PNA into the channels of AlPO4-5 with the nitro group first caused by the intrinsically higher affinity of the AlPO4-5 channels to nitro than amino group.16 Since the sizes of the crystals far exceeded (such as 130 μm) the wavelength of the incident laser beam (1.064 μm), the polarization reversal that occurred at the center of each crystal did not affect the overall SHG activities of the dye-loaded AlPO4-5 crystals. They also found that the ALPO4-5 crystals loaded with 4-nitro-N,N-dimethylaniline17 or (dimethylamino)benzonitrile18 are active for SHG.
The MFI-type structures such as ZSM-519 and Sb-incorporating silicalite-1 (Sb-SL)20 [centrosymmetric (Pnma) zeolites having a three-dimensional channel system consisting of straight 0.54×0.56 nm channels in one direction and sinusoidal 0.51×0.54 nm channels in other direction perpendicular to the straight channels] have also been shown to be SHG active upon PNA loading. However, in the case of Sb-SL loaded with PNA, the SHG activity disappeared after several exposures to incident laser beams. Unlike ZSM-5 and Sb-SL which contain Al and Sb, respectively, in the framework, the closely related pure silica ZSM-12 [a centrosymmetric (C2/c) silica zeolite having one-dimensional channels with the diameter of 0.56×0.59 nm] did not show any SHG activity even after inclusion of PNA.21 
Interestingly, PNA-loaded mesoporous silica, MCM-41 (an amorphous silica having a hexagonal array of channels with the diameter of 2-8 nm) also showed a SHG activity, although its pore diameter is much larger than the size of PNA.22 For them to be SHG active, however, aging of the composite under humid air for several weeks was necessary, indicating the requirement of the water-assisted secondary reorganization of the included PNA molecules to give rise to a net bulk dipole moment.
Thus, the previous works have demonstrated the potential of zeolites and the related nanoporous materials to be developed into versatile inorganic hosts for preparation of practically viable organic-inorganic composite SHG materials. However, the previous studies have been limited to PNA (βPNA=34.5±4×10−30 esu) and several analogous DNLO molecules with β values lower than 500×10−30 esu23,24 and a large variety of DNLO molecules with much higher β-values have been known to be not aligned spontaneously upon incorporating them into zeolite pores. Accordingly, the spontaneous alignment of PNA and several analogous DNLO molecules upon entering AlPO4-5, Sb—SL, and ZSM-5 channels has been determined rare cases.
Furthermore, the examined zeolite forms have been limited to powders and very small single crystals that bear no practical applicability. It is therefore necessary to develop methods for incorporating general DNLO molecules with higher β values into the zeolites in one direction. Also, for practical viability, efforts should be directed at extending zeolite hosts for the aligned incorporation of DNLO molecules to microcrystals, thin films of mono- or multi-layered microcrystals on substrates that are uniformly aligned, films prepared by growing microcrystals uniformly aligned on supporting substrates and supercrystals prepared by aligning microcrystals in three dimension with no substrates.
Throughout this applications various publications are referenced and citations are provided in parentheses. The disclosure of these publications in their entities are hereby incorporated by references into this application in order to more fully describe this invention and the state of the art to which this invention pertains.